[CURATOR-654] DistributedBarrier watcher leak

The DistributedBarrier adds a zookeeper exists watcher in the waitOnBarrier() method. It appears as though the watcher is never removed. In a system where DistributedBarriers are used heavily this eventually results in the system running out of memory.

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Originally reported by sscott, imported from: DistributedBarrier watcher leak

• status: Open
  
• priority: Major
  
• resolution: Unresolved
  
• imported: 2025-01-21
  

Locate DistributedBarrier and its waitOnBarrier() method, then inspect how the ZooKeeper exists watcher is registered and what lifecycle follows. Reproduce repeated barrier use and verify that completed barriers no longer cause watcher or memory growth.

Written by the indexing model from the issue text.